Until now, so far as the inventors are aware, most computer networking systems did not allow for stub or star connections in the transmission lines. Users wishing to connect another computer midway along an existing line would have to "stretch" the existing line back and forth to the new computer to avoid having a stub connection. This could mean that excessive cable runs would be necessary to connect the new computer and that the maximum distance limit of the LAN cable may be exceeded.
Some local area networks, such as Appletalk.TM., do allow for a star connection of up to 4 lines. However, systems with more than 4 computers would also have to run cable back and forth to connect the additional machines along the length of each line.
Ethernet does not allow for any star connections. Even one star connection is enough to plague it with errors and halt the network's activities. A reflection is generated by the line impedance discontinuity occurring at the star connection point and this returns part of the transmitted information to the sending computer. This erroneous information will cause the system to fail.
Ethernet also uses the average dc voltage on the coaxial line to determine if a collision has occurred. If the stub is terminated with a resistance, the average dc level will change due to the increased loading. This will cause the collision detection to fail. It should also be noted that dc power cannot be transmitted down an Ethernet line for use by any circuits connected to the line. Therefore, any circuit connected to the line would have to be connected to its own external power supply which would render it sensitive to local power outages.
An active device based on logic circuits may be used in a LAN to provide the function of a star connection. In Appletalk.TM. networks, active hubs may be used when more than 4 devices must be connected in a star configuration. Commercially available active hubs may have 12 or 24 ports and will take data received on one port, regenerate the waveform for signal levels, then output it to all the other ports. This regeneration solves the signal distortion problem caused by passively starring lines together, however, there is considerable cost associated with the required circuitry.
In a similar application, Ethernet networks using twisted pair wiring (10BaseT) must use active hubs to connect devices together. The 10BaseT specification does not allow any wires to be starred together. The active hub works like Appletalk hub in that it receives a signal on one port, recovers the data from the signal, and outputs a regenerated signal on all ports.
A star connection also exists in voice telephony systems and is known as a conference circuit. Active conference circuits based on logic circuits are commonplace in the modern digital network. Conference circuits can also be formed on analog subscriber lines with branch connection of extension line stations within a residence. It should be noted that only a narrow band of frequencies is required for voice communication and that reflections caused by the discontinuity at the branch connection are tolerated by the users.